The synthesis and non-linear optical properties of a series of Icosahedral carboranes are described. The carboranes are substituted on both carbon atoms, with a series of electron-donating and electron-withdrawing substituents. The single-crystal X-ray structure of two of the series 1,2-bis(3-nitrophenyl)-1,2-dicarba-closo-dodecaborane(12), 1, and 1-(3-nitrophenyl)-2-(4-nitrophenyl)-1,2-dicarba-closo-dodecaborane, 3, are presented. 3 crystallises in the space group P, with four molecules per unit cell with cell dimensions of a= 9.706(1)Å, b= 13.798(2)Å, c= 17.99(2)Å, α= 109.17(1)°, β= 100.86(1)° and γ= 101.78(1)°. 1 crystallises in the space group P, with two molecules per unit cell with cell dimensions of a= 7.585(2)Å, b= 11.032(3)Å, c= 12.252(4)Å, α= 105.29(3)°, β= 90.30(3)° and γ= 96.46(2)°. The second-harmonic generation (SHG) properties of all of the compounds prepared are described, and are related to the ZINDO-SOS calculations of molecular polarisibility, β, computed for selected examples.